In the manufacture of semiconductor devices, ion implantation is used to dope semiconductors with impurities. Ion beam implanters are used to treat silicon wafers with an ion beam, in order to produce n or p type extrinsic material doping or to form passivation layers during fabrication of an integrated circuit. When used for doping semiconductors, the ion beam implanter injects a selected ion species to produce the desired extrinsic material. Implanting ions generated from source materials such as antimony, arsenic or phosphorus results in “n type” extrinsic material wafers, whereas if “p type” extrinsic material wafers are desired, ions generated with source materials such as boron, gallium or indium may be implanted.
Typical ion beam implanters include an ion source for generating positively charged ions from ionizable source materials. The generated ions are formed into a beam and directed along a predetermined beam path to an implantation station. The ion beam implanter may include beam forming and shaping structures extending between the ion source and the implantation station. The beam forming and shaping structures maintain the ion beam and bound an elongated interior cavity or passageway through which the beam passes en route to the implantation station. When operating an implanter, this passageway is typically evacuated to reduce the probability of ions being deflected from the predetermined beam path as a result of collisions with air molecules.
The mass of an ion relative to the charge thereon (e.g., charge-to-mass ratio) affects the degree to which it is accelerated both axially and transversely by an electrostatic or magnetic field. Therefore, the beam which reaches a desired area of a semiconductor wafer or other target can be made very pure since ions of undesirable molecular weight will be deflected to positions away from the beam and implantation of other than desired materials can be avoided. The process of selectively separating ions of desired and undesired charge-to-mass ratios is known as mass analysis. Mass analyzers typically employ a mass analysis magnet creating a dipole magnetic field to deflect various ions in an ion beam via magnetic deflection in an arcuate passageway which will effectively separate ions of different charge-to-mass ratios.
Dosimetry is the measurement of ions implanted in a wafer or other workpiece. In controlling the dosage of implanted ions, closed loop feedback control systems typically are utilized in order to dynamically adjust the implantation to achieve uniformity in the implanted workpiece. Such control systems utilize real-time current monitoring to control the slow scan velocity of an implanter. A Faraday disk or Faraday cup periodically measures the beam current and adjusts the slow scan speed to ensure a constant dose. Frequent measurement allows the dose control system to respond quickly to changes in beam current. The Faraday cup may be stationary, well shielded, and located close to the wafers, making it sensitive to the beam current dosing the wafers. However, Faraday cups measure only the electric current portion of the beam current.
Interactions between the ion beam and gases evolved during implant can cause the electric current, a charge flux, to vary even when the particle current, a dopant flux, is constant. To compensate for this effect, the dose controller may read the beam current from the Faraday cup and the pressure from a pressure gauge concurrently. When a pressure compensation factor is specified for an implantation recipe, the measured beam current is modified by software to present a compensated beam current signal to the circuit controlling the slow scan. The amount of compensation (e.g., in the compensated beam current signal) in such a closed loop system may thus be a function of both the current measured at the Faraday cup and the pressure.
When properly applied, pressure compensation improves repeatability and uniformity over a wide range of implant pressures. However, the vacuum in an implanter is never perfect. There is always some residual gas in the system. Usually the residual gas poses no problems (in fact, a small amount of residual gas is beneficial for good beam transport and effective charge control). However, at high enough pressure, for example, increased pressure due to photoresist outgassing, charge exchange between the ion beam and the residual gas can cause dosimetry errors. If the shift in dose between implants into bare wafers and implants into photoresist-coated (PR) wafers is unacceptably large, or if the dose uniformity is significantly degraded, then pressure compensation may be employed in order to improve uniformity.
Charge exchange reactions between ion beams and residual gas can add or subtract electrons from the ion, changing the ion's charge state from the value desired in the recipe. When the charge exchange reaction is neutralization, a portion of the incident ion flux is neutralized. The result is a reduction in the electrical current while the particle current (including neutrals) remains unchanged. When the charge exchange reaction is electron stripping, a portion of the ion flux loses electrons. The result is an increase in the electrical current while the particle current remains the same.
For typical recipes where charge exchange is an issue, the beam often undergoes much more neutralization than stripping. As a result, the beam current measured by the Faraday cup decreases whenever the end station pressure increases. Ions in the beam are neutralized, but they are not deflected or stopped by the residual gas. The dose rate, dopant atoms per area per time, is unchanged by charge exchange after the analyzer magnet. Implanted neutrals contribute to the dose received by the wafer, but are not measured by the Faraday cup. As a result, the wafer may become overdosed.
Pressure compensation may thus be employed whenever charge exchange between the ion beam and residual gas in the process chamber has a significant effect on dose. The pressure at which this happens depends on the recipe and the process specifications. For some recipes, compensation is required to meet implanter specification when the pressure due to photoresist outgassing is 5×10−6 torr as measured on the pressure gauge. For most recipes where the pressure due to photoresist outgassing is 2×10−5 torr or higher, compensation may be worth investigating. Such compensation may include measuring the effect of photoresist outgassing by implanting monitor wafers with and without photoresist, and comparing the measured variation to the process specification. The amount of compensation required depends on the pressure, which the dose controller reads from a pressure gauge during the implant.
In addition, changes in the ion source output itself may result in some of the beam current variations measured at the dose cups. Dose cup measurements of such ion source changes at the wafer are also subject to the proportion of neutral generation to the electric current measured and outgassing pressure changes as discussed. It is necessary to compensate dose rates for the actual change in ion flux at the wafer which requires the system to differentiate between a change in the current caused by a change in source output and a change caused by charge exchange in the gas in the beam path. Therefore, the use of such dose cup measurements to correct or compensate dose rates may be significantly hampered by these variables.
Thus, there is a need for improved systems and methods for obtaining uniform dose rates in ion implanters without the added complications and costs associated with the use of pressure measurements and pressure compensation in the presence of beam current changes from the ion source and outgassing from the wafer.